Among the power system corrective controls, defensive islanding is considered as the last resort to secure the system from severe cascading contingencies. The primary motive of defensive islanding is to limit the affected areas as soon as possible to maintain the stability of the resulting subsystems and to reduce the total loss of load in the system. The design of the defensive islanding must address the questions of where and when to island the system to ensure minimum impact and the recovery of the affected portion of the system. In this paper, the critical islanding time is explored as a security index for applying the defensive islanding where the boundaries of the islands are decided by using the slow coherency concept between generators. The computations are performed on a test system to specify the maximum allowed time for operators to apply islanding before falling in blackout due to some critical contingencies.